1. Field of the Invention
This invention generally relates to disk drives, and more particularly to an optical disk drive having the function of detecting the sector mark and the address of a sector recorded on an MCAV optical disk.
2. Description of the Prior Art
A various kinds of recording formats for the optical disk drive have been proposed in order to increase the disk memory volume and in order for information to be reliably recorded on or retrieved from a target address of the disk. These formats are CLV (Constant Linear Velocity), CAV (Constant Angular Velocity), MCLV (Modified CLV), and MCAV (Modified CAV) when classified according to their sector arrangement. MCLV is a CLV with the rotation rate control of the spindle motor being simplified. MCAV is a CAV with its memory volume increased.
On a CAV disk, there are the same number of sectors in each track from the innermost side to the outermost side of the disk. Since the disk is rotated at a predetermined constant rate, the frequency of a signal retrieved from the disk should be constant over the entire disk. In order for information to be recorded on or retrieved from the disk, a sector mark located at the beginning of each sector is detected by sampling a retrieved signal with a reference clock of a predetermined frequency, and a read clock is locked by means of the PLL (Phase Locked Loop) and the like to a signal retrieved from a portion called VFO, which follows the sector mark within the sector. Then, the sector address recorded in the portion following VFO is read by using the read clock which has been already locked, and thereby the sector address is detected. The sector mark is a mark for indicating the beginning of the sector and must be able to be detected by a reference clock of a predetermined frequency so that the sector mark has a longer mark length and a stronger signal output or is recorded with a different pattern than other signals. The signal recorded on VFO is a data pattern designed to be used for locking a read clock to a retrieved signal frequency so as to ensure that signals can be read even if there are variations in the disk rotation rate.
For both the CAV disk and the MCAV disk, the portion at the beginning of a sector comprising the sector mark, VFO, the sector address, and etc., is called address portion of the sector, and the part of the address portion other than the sector mark is referred as ID area. Typically, there are a plurality of the same ID areas following a single sector mark in the sector to make sure that the disk drive can read the sector address without errors.
In the contrast to the CAV disk, the number of sectors in one track on an MCAV disk gets larger as the track gets nearer to the perimeter of the disk, which can achieve an increase in memory volume of the disk. FIG. 1 shows the sector arrangement on a disk which is formatted according to MCAV. A spiral shape track 12 is formed on a disk 11, and there are a different number of sectors for each zone, i.e., a zone 13a, a zone 13b, . . . , with the total number of the zones on the disk being several to several dozens depending on the type of the disk. Since a disk for MCAV is rotated at a predetermined constant rate, a read clock frequency for recording and retrieving is different for each zone, and should be increased progressively as a zone gets nearer to the perimeter of the disk.
In order to start up an MCAV disk, i.e., to make an MCAV disk ready to record or retrieve information, with a CAV type disk drive, the disk drive moves its head to a predetermined position on the disk, and starts reading a zone of this known position with a read clock of a predetermined frequency. Another method of starting up an MCAV disk with a CAV type disk drive is to progressively change the read clock frequency, and to start reading at the point where this read clock frequency corresponds to the frequency of signals recorded on the disk. These methods, however, cannot start up a disk and detect the address of a sector within such a short period of time that the address can be detected at the first sector encountered.
Accordingly, there is a need for a device in the field of the MCAV disk drive which can detect the address of the first encountered sector by reducing the time required for starting up the disk.